Epidermal Growth Factor Receptor (EGFR), also known as HER1 or ErbB1, is a member of the type 1 tyrosine kinase family of growth factor receptors. These membrane-bound proteins possess an intracellular tyrosine kinase domain that interacts with various signaling pathways. Upon ligand binding, receptors in this family undergo dimerization and subsequent autophosphorylation of the tyrosine kinase domain. The autophosphorylation triggers a cascade of events in intracellular signaling pathways, including the Ras/MAPK, PI3K and AKT pathways. Through these pathways, HER family proteins regulate cell proliferation, differentiation, and survival.
A number of human malignancies are associated with aberrant expression or function of EGFR. (Mendelsohn et al., (2000), “The EGF receptor family as targets for cancer therapy,” Oncogene, 19:6550-6565.) In particular, it has been demonstrated that some cancers harbor mutations in the EGFR kinase domain (exons 18-21). In non-small cell lung cancer (NSCLC), these mutations were shown to promote anti-apoptotic pathways in malignant cells. (Pao et al. (2004). “EGF receptor gene mutations are common in lung cancers from “never smokers” and are associated with sensitivity of tumors to gefitinib and erlotinib”. P.N.A.S. 101 (36): 13306-13311; Sordella et al. (2004). “Gefitinib-sensitizing EGFR mutations in lung cancer activate anti-apoptotic pathways”. Science 305 (5687): 1163-1167.)
Therapies targeting EGFR have been developed. For example, cetuximab (ERBITUX™) and panitumumab (VECTIBIX™) are anti-EGFR antibodies. Erlotinib (TARCEVA™) and gefitinib (IRESSA™) are quinazolines useful as orally active selective inhibitors of EGFR tyrosine kinase. These drugs are most effective in patients whose cancers are driven by aberrant EGFR activity. A randomized, large-scale, double-blinded study of IRESSA™ (IRESSA Pan-Asia Study (IPASS)) compared gefitinib to the traditional chemotherapy as a first-line treatment in non-small cell lung cancer (NSCLC). (Mok et al. (2009) “Gefitinib or carboplatin-paclitaxel in pulmonary adenocarcinoma.” N Eng J Med 361:947-957)). IPASS studied 1,217 patients with confirmed adenocarcinoma histology. The study revealed that progression-free survival (PFS) was significantly longer for IRESSA™ than chemotherapy in patients with EGFR mutation-positive tumors. The opposite was true for tumors where EGFR was not mutated: PFS was significantly longer for chemotherapy than IRESSA™. The study demonstrated that to improve a lung cancer patient's chances of successful treatment, EGFR mutation status must be known.
Analysis of clinical outcome revealed that patients with tumors harboring mutations in the kinase domain of EGFR (exons 18-21) have better response to erlotinib than those with tumors expressing wild-type EGFR. (U.S. Pat. Nos. 7,294,468 and 7,960,118) These mutations are predictive of response to tyrosine kinase inhibitors (TKIs) such as quinazolines erlotinib (TARCEVA™) and gefitinib (IRESSA™). Among the EGFR mutations, in-frame deletions and substitutions of nucleotides in the region of exon 19 including nucleotides 2235-2257 (corresponding to amino acids 746-753) is especially common in lung cancer patients (see U.S. Pat. No. 7,294,468 and Lynch et al. (2004) “Activating mutations in the epidermal growth factor underlying responsiveness of non-small cell lung cancer to gefitinib.” NEJM 350:2129.) These mutations are thought to result in an active kinase with altered properties, including increased susceptibility to inhibition. See Paez et al. (2004) EGFR mutations in lung cancer: correlation with clinical response to Gefitinib therapy, Science 304:1497.
Some mutations in the EGFR kinase domain are common, while others occur less frequently. However, it is essential that a clinical test for EGFR mutations target as many mutations as possible. This will assure that patients with rare mutations do not receive a “false negative” test result. If a rare mutation goes undetected, the patient with such a mutation will not receive potentially life-saving treatment. Therefore when a new mutation in the EGFR kinase domain is discovered, detecting this mutation has the potential of affecting the clinical outcome in some patients.